Corrosion is a leading cause of process equipment and piping failures in industrial water and process systems. Each year companies are forced to spend billions of dollars for corrosion-related equipment replacement costs and the associated production downtime. Galvanization is a protective zinc coating that is chemically bonded to a metal (usually iron or steel) surface. The galvanizing process produces a coating of zinc-iron intermettalic alloy layers on steel with a relatively pure outer layer of zinc. The zinc coating of the metal's galvanized surface performs as a sacrificial anode, which protects the steel and also forms a barrier to a continuous ongoing reaction of the steel with the environment. When galvanized steel is exposed to a neutral pH and moderately hard water environment a surface barrier of non-porous zinc carbonate/zinc hydroxide forms to prevent further rapid corrosion of the zinc coating. Because zinc is a very reactive metal, the protective “basic zinc carbonate” barrier must be formed for the HMG (hot milled galvanized) steel component to achieve full service life.
Zinc coating is used in a variety of applications and offers a certain degree of corrosion protection for the underlying metal by providing a mechanical barrier to the elements and environment as well as electrochemical resistance to corrosion. Several galvanizing methods exist, such as electroplating, continuous galvanization, hot dip, hot mill. In practice, hot dipped galvanized surfaces generally have a thickness of about 2.5 mils and hot milled galvanized surfaces have a thickness of about 4 mils. Many industrial water systems, such as cooling water circulation systems have such galvanized surfaces.
Galvanized steels typically found in refrigeration condensers or evaporative coolers can experience a type of premature corrosion, generally identified as “white rust.” The term “white rust” refers to a type of corrosion product affecting galvanized surfaces characterized as an accumulation of white, fluffy, or waxy non-protective zinc corrosion product that adheres to the zinc surface of galvanized steel. With this non-protective porous reaction product in place, the surface is not passive to future zinc reaction and rapid corrosion may continue. White rust is capable of causing considerable damage to the zinc coating and is also detrimental to the appearance of the galvanized coating. If left unchecked, white rust will continually corrode affected galvanized surfaces and eventually lead to early failure of the coating.
Moreover, white rust can become very destructive if allowed to advance untreated, because once the zinc layer has been consumed, corrosion of the mild steel may progress rapidly. In particular, pH levels less than 6.0 or greater than 9.0 will increase zinc corrosion significantly. Anions of sulfates, chlorides, and nitrates at high levels (as the ion) may accelerate loss of zinc, and the amount and type of alkalinity present is also important. High concentrations of free halogens such as chlorine (or bromine) may also be corrosive to zinc galvanizing. Low hardness water, calcium hardness as CaCO3 is <50 ppm in the recirculating water, may result in accelerated zinc corrosion. High levels of chemical treatments likewise are a contributing factor in the corrosion of zinc.
To ensure long service life, the galvanized surfaces in cooling towers typically must be allowed to “passivate” or form a protective barrier prior to initial operation or start-up. Proper water treatment and start-up procedures are also essential. One way to passivate the surfaces is to allow the zinc coating to develop a natural nonporous surface of basic zinc carbonate during initial start-up of the cooling tower. This natural chemical barrier helps prevent or slow further rapid corrosion of the zinc coating from the environment as well as from normal cooling tower operation.
This basic zinc carbonate barrier, believed to be a zinc carbonate/zinc hydroxide compound (as discussed in “Guidelines for Treatment of Galvanized Cooling Towers to Prevent White Rust,” published by the Cooling Tower Institute in June 1994) typically forms within eight weeks of initial cooling tower operation with water of neutral pH (i.e., pH 6.5 to 8.0) and moderately hard water environment. A typical solute content range would be calcium (CaCO3) content of 100 ppm to 300 ppm as bicarbonate alkalinity and about 100 ppm CaCO3 hardness. Formation of the protective zinc carbonate barrier is important for the cooling tower to resist further corrosion. Barrier absence could result in severe white rust formation and have a significant negative impact on the service life of a cooling tower.
White rust is also a form of zinc carbonate that has a different porous structure, rate of formation, and density than the protective zinc carbonate barrier described above. If the water hardness levels, measured by CaCO3 hardness, reach levels below 50 ppm (i.e., soft water), accelerated zinc corrosion generally results. Certain ionic content in the water, such as sulfates, chlorides, and nitrates at levels greater than about 250 ppm may also contribute to accelerated zinc corrosion. Thus, routine inspection of the cooling tower coupled with adequate control of the water chemistry aids in the prevention of white rust formation.
Current white rust corrosion prevention programs include a combination of pre-passivating the cooling tower combined with ongoing water chemistry management to support the viability of the passivation layer. In addition to the basic zinc carbonate protective layers, as described above, white rust preventatives include pretreatment with inorganic phosphate and chromate passivation. Such inorganic solutions have limited effectiveness and are steadfastly becoming the object of federal and local regulations due to environmental concerns.
Other solutions for white rust prevention include using selective thiocarbamates, organo-phosphorous compounds, and tannins to passivate the surface. For example, U.S. Pat. No. 5,407,597 provides a formulation including a mixture of an organophosphorous compound, a thiocarbamate compound, and soluble metal salt compound. The components of this formulation are used as a combination and the ingredients tested alone typically do not control white rust formation. The formulation in U.S. Pat. No. 6,468,470 B1 includes a multi-component system of an organophosphorous compound, a tannin compound, and a soluble salt of a metal.
Moreover, under normal operating conditions, cooling towers have substantial evaporative water loss. As a result, large quantities of “make-up” water are introduced into the system that commonly contains ionic species, such as calcium, magnesium, sulphate, and chloride. Increased alkalinity (e.g., carbonate, bicarbonate, and hydroxide ions) may also cause white rust corrosion. Particularly, accumulation of carbonate alkalinity, with a concomitant pH increase, creates an ideal white rust-forming environment. This accumulation is one of the major causes of white rust. The presence of excess anions and/or soft water can aggravate the degree of white rust formation by, for example, reacting with the zinc coating to produce zinc hydroxide.
As an integral component of cooling water circulation systems biocides are essential is preventing algal, bacterial, and fungal contamination of the systems. Some of these biocides sometimes promote white rust formation as a byproduct because they chemically react with certain white rust inhibitors and/or with the zinc coating. For example, sodium hypochlorite (i.e., bleach) is a common biocide and is highly reactive.
Because high pH levels are also contributing factor to white rust formation, the addition of a sufficient quantity of free acid, commonly sulfuric acid, to the cooling water helps preclude the formation of white rust. Such free acid addition creates concerns for those handling the free acid and also creates potential for metal corrosion from the acid itself due to overfeed or spillage. None of these passivation or maintenance procedures described above provides a complete solution to the white rust problem. There thus exists a need to provide efficient and improved compositions and methods of reducing white rust corrosion.